Viewfinder Optics Part I

A viewfinder is a system that allows the photographer to view the scene before the photo is actually taken. It is required to get a preview of the photo to find the perfect composition and perspective.

On some compact digital cameras, a separate viewfinder can be used in addition to the live-view function on the LCD display. These separate viewfinders use an own lens array and generate a view of the scene that is very close to what the sensor will perceive for the original photo. However, as they are applied in a slightly different position than the main lens, they are likely to suffer from parallax error. The parallax error occurs when an object placed in front of a background is seen from different positions. The slightly different angle inherent with another position will sometimes result in a very different impression of the scene. Applying this principle to photography, it means that the scene may actually look nice on the separate viewfinder but the final image doesn’t exactly turn out as expected. You can easily comprehend the parallax error if you raise one arm and turn up a thumb. Then, close the right eye and look at the composition of background and your thumb representing a subject to be photograhed. Then, look at the scene with the right eye only while not changing the position of your arm and thumb. You will notice that the composition of background elements and your thumb has changed, eventually resulting in background elements to be covered by the foreground or large gaps between the subject and other objects. Also, a separate viewfinder always provides the same perspective of the scene regardless of the type of photo-taking lens used on the camera.

In digital single lens reflex cameras, there is no additional lens used for the viewfinder, hence the name single lens. A viewfinder in a DSLR camera includes various units of lenses, prisms and displays. As described in the Internal Structures article, incident light from the main lens is being deflected towards the viewfinder optics by the reflex mirror. This conversion of incident light is the reason why DSLR cameras do not suffer from parallax error and the photographer can view a scene that does not perceptibly differ from the scene the sensor will record. In addition to the advantage described, there are some more supplementary functions in a viewfinder that will be illustrated in this article.

The Pentaprism

A pentaprism is a key element for the correct representation of the viewfinder image. The name results from it’s five-sided shape and it’s primary function as a reflecting prism. Although it provides five sides, there are only two sides used for reflections while two sides are required to let light in and out. One last side is not optically used but is kept flat for the sake of compact size.

In a regular prism, light can enter on one side but finally exits the prism on another side. In a reflecting prism, light will not be allowed to exit so that it deflects within the prism. The reflections inside the prism are not caused by total internal reflection that would occur if the beams were incident at an angle less than the minimum angle for total internal reflection (critical angle), but instead the two faces are coated to provide mirror surfaces. The image below shows how the incident beam reflects inside a regular prism twice.

As the prism in a DSLR camera receives a copy of the original image rendered by the lens, there is one problem. The camera lens produces an image that is both vertically and horizontally reversed. Some detailed descriptions of image formations by lenses can be read later in the lens articles. By the reflection of the reflex mirror, this image gets re-inverted vertically, leaving an image horizontally reversed. In this situation, the image needs to be reflected left-to-right as the pentaprism transmits the image. This horizontal inversion is done by replacing one of the reflective faces of a normal pentaprism with a section shaped like a roof, two additional surfaces angled towards each other in a 90° angle, which horizontally reverses the image back to normal. The image below shows both features of a roof pentaprism, the deflection and correction of the image.

Depending on the value of a DSLR camera, their roof pentaprisms come in different designs.

Most entry-level DSLRs are equipped with pentamirrors. These pentamirrors are not made of solid glass but a hollow case in the shape of a pentaprism with thin mirrors applied inside. The advantage of pentamirrors is that they are light and inexpensive, but they typically do not provide the same brightness of viewfinder images known by pentaprisms. This can be a critical factor if manual focusing needs to be done under some low-light conditions. Given the fact that they consist of a small chamber, there is also the risk that dust can enter if no perfect sealing is applied, degrading the mirrored surfaces over time.

Advanced- and pro-level DSLRs are typically using pentaprisms in an all-glass design with specially coated surfaces. They are more expensive and have a higher weight than pentamirrors, but they deliver more light to the eyepiece thanks to their more efficient internal reflections. Independently of its material and type of reflection, pentaprisms can differ in another respect. There are different viewfinder coverage ratios among DSLRs. While a Canon EOS 60D (mid-range camera) only provides a viewfinder coverage of 96%, the Canon EOS 7D (advanced-level) provides a viewfinder coverage of 100%. This means that the viewfinder of a 60D does not show 4% of the corner areas while the 7D shows an identical, uncropped scene. This does not sound like a real difference, but it can require post-processing if an image suddenly shows unexpected elements in the corner areas that have not been seen on-camera. This viewfinder coverage is determined by the dimension of the pentaprism/pentamirror and is dependent on the camera model.

Focus Screen

The purpose of a focusing screen is to get a realistic preview of an image that is being formed by the main lens. Only with a focusing screen, also referred to as ground glass, the viewfinder will reveal how the final image will be captured by the sensor. In addition, focusing screens often include useful features to improve manual focusing accuracy. To benefit from different designs of focusing screens, these are often interchangeable on some camera models. These properties of focusing screens will be shown in this section of the article.

Ground Glass

The focusing screen mainly serves to intercept light rays on their way from the main lens towards the pentaprism, deflected by the reflex mirror inbetween. Therefore, the top surface of the focusing screen is roughened to act as a ground glass. Canon calls this side of the focusing screen laser matte surface. If no ground glass was used at all, an image could still be perceived through the viewfinder, but it would not allow the photographer to assess whether a particular object will be in focus on the camera sensor or not. The viewfinder would rather display a three-dimensional-copy of the scene where it would be impossible to adjust focus for the camera.

In order to display this intermediary image from the perspective of the camera sensor, the focusing screen must be located in the focal plane of the main lens. In other words, incident light from the photo-taking lens must travel the same distance towards the ground glass than towards the camera sensor. As the reflex mirror deflects incident light towards the upper part of the camera while in viewfinder mode, the ground glass typically is located under the pentaprism to fulfill this requirement.

This setup lets the camera produce an image of the scene on the ground glass. The important characteristic of the focusing screen is that it’s surface is matte yet translucent so that it will emit light again towards the pentaprism for observation by the photographer. Due to the matte surface, out-of focus objects will be displayed blurred while objects in focus will be displayed sharply. This is exactly what is required to get a realistic 2D-preview of the scene.

Focusing Aids

UNDER CONSTRUCTION

Accessory Focusing Screens

A particularly helpful feature of many DSLR cameras is that they use interchangable focusing screens. Most DSLR cameras are equipped with a standard focusing screen includig the matte surface and focusing points. These standard focusing screens do frequently not longer use split-image focusing aids or microprism fields as autofocus has become extremely precise and reliable on today’s cameras and manual focusing has lost significance. However, in some situations, manual focusing is still required and if a photographer needs some more control on sharpness, it can be helpful to exchange the standard focusing screen with a specialized accessory screen including split-image focusing aids and microprism fields.

Furthermore, some other accessory focusing screens consist of a matte surface with corner markings or guidance lines engraved. These framing aids can be extremely useful for microscopic or architectural photography. It should be noted at this point that some DSLR cameras have an additional LCD panel directly adjacent to the focusing plate. This fully translucent LCD panel is designed to display grid lines on demand so that the photographer will not always have to interchange camera hardware.

Condenser Lens

One disadvantage from the ground glass – being a typical diffuse surface – is that light incident by an acute angle will be emitted in a much larger angle when forwarded towards the viewing system. In the focal plane, the intensity of an entire light flux is condensed into one tiny spot. After re-emission towards the viewing system, this intensity of light is now distributed over the large angle. Accordingly, the light intensity will be reduced at the eyepiece because some light is diverted from its original direction so that it will not reach the eyepiece at all.

The image below illustrates how the ground glass affects the intensity of light. In the interest of simplification, assuming that the eyepiece is in line with the incident light from the main lens (which in reality is not the case), it becomes obvious that only a reduced amount of light is captured by the viewing system (pentaprism, eyepiece lenses) resulting in a reduced viewfinder brightness.

To increase light intensity in the viewfinder, a condenser lens is typically implemented between the ground glass and the pentaprism. The condenser lens serves to “collapse” the cone of rays from any little spot on the ground glass so that more of that light finds it’s way through the viewing system to the eye. The image below shows the effect of a condenser lens on the brightness of the viewing system.

Another implementation of a condenser lens is a fresnel lens (named after it’s inventor) that is used in place of the usual convex lens due to weight and space factors. By its special design, a fresnel lens performs the same function as a convex lens while being considerably smaller and therefore makes the camera lighter and more compact. A fresnel lens takes advantage of the fact that on conventional lenses, the refraction of light only occurs on the curved surface of the lens, provided that the incident light does not hit the surface in a 90 degree angle. If a particular convex lens is having an entirely flat surface on the left and a curved surface on the right, most of the optical material in a conventional lens does not contribute to the refraction of light if illuminated from the left (in a 90 degree angle) and can be removed, as shown in the illustration below. The reduction of “unused” material creates little steps that can be observed from the front of the lens and will show as several rings. To prevent the photographer from seeing the rings inherent with the fresnel design, the spacing between ring peaks on a fresnel lens is significantly small. In modern DSLR cameras, this spacing is under 0.04 mm. Consequently, it is nearly impossible to see the rings in the focusing screens with the naked eye.

The downside of a fresnel lens is that it has a slightly reduced image quality. This is not a problem for the viewfinder system, but the actual photo-taking lenses still have to rely on conventional bulky lenses. Due to their very flat layout, these fresnel condenser lenses are typically integrated into the focusing screen directly as shown in the image below.

It may sound unprofitable to invest so much technology into the brightness of the viewfinder, but this is crucial due to the fact that also the central light metering system is located within the viewfinder optics and it is vital for the metering sensor to get as much light as possible.

Focus Indicators

Probably the most useful piece of viewfinder information is the information on whether the image is in focus or not. Virtually any digital camera has some kind of focus indicator. The most common method is to display individual focus indicators – either points or small rectangles (AF frames) or a combination of both – on different positions within the preview screen. The positions of the focus indicators are typically pre-defined and depend on the camera model. Entry-level DSLR cameras usually only provide a limited amount of indicators (up to 10) while top of the line series models today provide a numerous set of indicator spots spread over the entire preview screen (over 60 focus indicators on the Canon EOS 7D Mark II, released in Sep 14).

Also depending on the camera model and the technology used, an in-focus situation is indicated by either a beep sound of the camera or the active focus point lighting up in the viewfider. Including some older technologies, there are various concepts for displaying and highlighting focus indicator points:

I – Engraved Focus Indicators

In many DSLR cameras, an older technology is applied. The focus indicators are directly engraved into the focusing screen surface. They merge with the image formed on the ground glass and can be seen in the viewfinder as pin-sharp markings. Although these indicators are etched into an otherwise transparent material, light from the photo-taking lens is totally deflected by these engravings so that these appear to be black. An alternative for engravings in the focusing screen is the use of a separate glass plate – called superimpose (SI) screen – located directly above the focusing screen and providing the engravings in this additional surface.

The Canon EOS 1000D uses a focusing screen where the rectangles are etched on one side of the glass plate and the points on the opposite side. As the focusing screen has a certain thickness and the squares are located precisely in the focal plane, as a result the points are slightly off the focal plane and do therefore not appear
as sharp as the rectancles when observed through the viewfinder.

Superimpose Concepts

With all focus points permanently engraved in the ground glass or the superimpose screen, they do always show up in the viewfinder. In order to confirm a sharply focused subject to the photographer, the camera has to highlight the individual focus points. Unfortunately, this highlight feature is not installed as simple as one could believe. Due to the implementation of the indicator points as fine engravings, these cannot light up on their own as they are no electronic components. On the other hand, self-illuminating components such as light emitting diodes (LEDs) require power supply structures that could hardly be concealed when running through the transparent ground glass. To prevent the ground glass to be interrupted by circuit paths, camera developers have come up with different approaches. Here are two implementations of highlight systems for engraved focusing points:

Rear Illumination

The rear illumination looks back on a relatively long history. This technology has already been used for analog cameras in the 1980s and was later replaced by the Superimpose LCD. In a viewfinder with rear illumination design, the superimpose array is applied to the rear surface of the pentaprism. This superimpose array consists of light emitting diodes (SI LEDs) and a prism (SI Prism). Light from an LED is guided through the SI Prism to the rear surface of the pentaprism. As light continues to travel through the pentaprism and through the condenser lens, it will eventually illuminate the focus point engraved into the focusing screen. The focus point in turn will reflect the light towards the viewfinder optical system where it will be directed towards the eyepiece. This effect is what the photographer will perceive as a glowing focus point.

In viewfinders of these early cameras (for example Canon EOS 1N, 1994), the focusing points are usually arranged in a horizontal line in the middle of the focusing screen. Due to this simple layout, the SI LEDs can also be arranged in a vertical strip. A pinhole mask is applied in front of the LEDs to minimize their beam angles in order to make sure that each LED only illuminates its intended focus point. The illustration below shows the superimpose array of the Canon EOS 1N analog SLR that includes five focus points in a row. It should be noted that this lateral cross section can only display the middle LED illuminating the middle focus point. The remaining LEDs as well as the remaining focus points are located outside (2) and inside (2) the drawing plane.

Superimpose LCD System

A slightly newer technology is the use of a superimpose LCD. This technology also requires the focus indicator points or AF frames to be engraved in the ground glass. In contrast to the rear illumination, the idea of the superimpose LCD System is not to have the focus indicator points reflect light from a light source behind the pentaprism, but to create the impression to the photographer as if they lit up directly.

The Superimpose LCD System is located in close proximity to the pentaprism and includes a light source (SI-LED) and a transmissive liquid crystal display (SI-LCD). The LCD acts like a digital mask that can either block all light from the LED or allow certain areas to become transparent. When a particular focus point is active, the LED emits light and the LCD partially turns off to let a small cone of light through. A mirror reflects light cones from the LCD into a downward direction where a light combining unit will again reflect them towards the eyepiece, effectively superimposing the viewfinder image with the image of the LCD. The illustration below clarifies the configuration of a superimpose LCD System.

Superimpose Lenses

The cone of superimposing light travels through an array of various lenses. A set of condenser lenses (dome lens I, dome lens II) is arranged to put the light source and the photographers eye into a conjugate image forming relation and thus acts to cause a light cone to efficiently enter the eye of the photographer. To be precise, the image forming relation between the light source and the eye of the photographer is determined by a composite condensing power of the condenser lenses and two more lenses used as projection lenses (SI lens I, SI lens II).

Liquid Crystal Display (LCD)

The liquid crystal display is primarily responsible for the superimposing effect. The display itself is a thin structure of various transmissive sheets of glass with a layer of liquid crystals inbetween. An additional layer of glass substrate directly adjacent to the liquid crystals includes electrodes that can turn the liquid crystals into an opaque surface by applying an electric field. The electrodes on the substrate are applied exactly in the same shape and configuration as all the focus indicators (points or frames) on the focus screen of the individual camera, just reduced in size. In order to light up a focus indicator point in the viewfinder, the LED quickly turns on and a light cone condensed by the dome lenses illuminates the backside of the liquid crystal panel. At the same time, the LCD largely turns opaque, blocking all light from the light source except for a tiny portion representing the active focus indicator point. In fact, this represents a negative image of a single focus point on the ground glass. This creates a very discrete cone of light travelling towards the light combining unit.

Light Combining Unit

A light combining unit is either composed of a dichroic mirror or a dichroic prism. It is located where the optical paths from the Superimpose Screen and the viewfinder intersect. This unit serves to flip the light cone from the Superimpose LCD System into a direction parallel to light from the viewfinder. This arrangement creates the illusion of the Superimpose LCD to be exactly in the position of the focus screen. Light from the Superimpose LCD is virtually combined with light from the viewfinder image, hence the name. As the LCD is a lot smaller than the camera’s focus screen, the condensing power of the projection lenses and that of the eyepiece lenses are combined to show the LCD in an enlarged state to the photographer.

The light combining unit takes advantage from the physical properties of dichroism. While a detailed description of dichroism would go beyond the scope of this article, the general principle can be explained as follows: A regular mirror usually has a reflectance of 100%. A semi-transparent mirror – also referred to as half-mirror or beam splitter – can for example be designed with a reflectance of 40% and a transmittance of 60% accordingly. Such a half-mirror can theoretically be used as a light combining unit but at the cost of a significant loss. If light from the Superimpose LCD was to shine on the half-mirror, only 40% could be reflected towards the eyepiece. However, much worse is the fact that only 60% of the viewfinder image could pass through the mirror, reducing viewfinder brightness for 40%. Both the viewfinder image and the information display by the liquid crystal panel would become extremely dark.

In contrast to a regular semi-transparent mirror, a dichroic mirror only has a very efficient reflectance (approx. 100%) for a very short range of wavelengths while it is highly translucent (approx. 90%) for other wavelengths. For the Superimpose LCD Systems, a dichroic unit typically reflects wavelengths around 700nm (red color) and keeps other wavelengths passing through. With the Superimpose LED having its emission center around that wavelength, this light is almost fully reflected by the dichroic unit to be guided to the eye of the photographer. On the other hand, about 90% of the viewfinder light coming from a subject image formed on the focus screen passes through the dichroic mirror to be guided also to the eye of the photographer. This physical property of dichroic units is akey to attain a bright viewfinder image and still have complex shapes superimposing the screen.

II – Intelligent Viewfinder Display

An intelligent viewfinder display represents a new concept used in DSLR viewfinders. In contrast to the other technologies shown, the intelligent viewfinder does not rely on focus points permanently etched into the ground glass but to have variable focus indicator frames that can be custom-tailored without changing the focusing screen. Furthermore, intelligent viewfinders can superimpose several other supplementary information onto the viewfinder image on demand.

The Canon EOS 7D was the first DSLR camera to be released with an intelligent viewfinder to provide a customizable overlay screen. This LCD panel can display a wide range of information such as focus points, frames and other shapes if required. At the same time, it can disable all shapes currently not used by the camera. This ensures that the intelligent viewfinder does not always show its full range of indicators but only these relevant for the photo. For example, the 7D has a set of 19 AF frames but does not display them all simultaneously as they would cover certain areas of the viewfinder image. The LCD can also display architectural grid lines, but these are not active by default. The new Canon EOS 7D Mark II even has a digital level display included into the viewfinder as well as other photo-taking information. In low light conditions, the intelligent viewfinder can light up AF frames and other indicators to enhance its visibility.

The dynamic overlay is attained by a transparent layer – composed of an LCD panel – applied directly above the focusing screen. In general, this LCD has the same structure than the LCD used for the superimpose LCD System except that the intelligent viewfinder LCD is configured to display a positive image of the information. Also, the intelligent viewfinder LCD is much larger in size than the superimpose LCD as it needs to cover the size of the ground glass. The image below shows the layout of an intelligent viewfinder as used in Canon DSLR cameras.

To light up information in the intelligent viewfinder, a different approach is applied. Unlike the previous superimpose systems, the intelligent viewfinder has an array of red LEDs applied directly at two sides of the transparent layer, leading light to the inside of the LCD from these positions (edge lit LCD). With no AF frame displayed, light cannot exit through the large LCD surfaces due to internal reflections. However, as soon as an indicator item is displayed, light will hit the crystals and light them up. These glowing LCD crystals in turn will emit light perpendicularly towards the large LCD surface, suddenly allowing it to exit the LCD. This light reflected by the LCD crystals travels through the viewfinder system in the usual way and can be observed by the photographer as viewfinder information lighting up. The image below summarizes the general concept of an intelligent viewfinder.

The LCD overlay does require a tiny amount of electrical power to operate. This is obviously no concern when the camera is turned on, but if the battery is removed the transmissive LCD suddenly loses a lot of brightness and contrast. This is perfectly normal and will return to full brightness once a battery is reinstalled in the camera (the camera doesn’t have to be turned on – it only requires a functioning battery pack to draw power for proper viewfinder operation).

Concluding to all focus indicator systems, it has to be noted that viewfinder systems typically include focus indicators such as AF points or AF frames, but these do only serve the photographer to know whether or not the subject is in focus. These focus indicators no not detect any focus or out-of-focus situation, they only display the result. For the focus detection, DSLR cameras rely on complex sensor units to let the camera lens find the perfect focus position. A detailed description on focus detection systems can be read in the focus article.